In many applications it is desirable to have high gain amplifiers, and at the same time have a linear transfer characteristic to prevent distortions of the signal to be amplified. An architecture of a differential amplifier commonly used in communication systems, in which these requirements are particularly desired, is a Darlington differential amplifier as shown in FIG. 1.
The illustrated Darlington differential amplifier is formed with bipolar transistors. The bipolar transistors may be replaced with MOSFETs and the following discussion still holds.
The amplifier is substantially formed by a pair of Darlington transistors QD1, Q1, QD2, Q2. Each pair includes an input transistor QD1 (QD2) controlled by a voltage Vin+ (Vin−) of a differential signal to be amplified. The input transistor QD1 (QD2) controls another transistor Q1 (Q2) connected in cascade. The collector or drain nodes in common of the two Darlington transistors QD1, Q1 and QD2, Q2 that form the output nodes of the amplifier are coupled to a supply line VDD through respective load resistors RC1, RC2. The Darlington transistors are coupled between them through two identical emitter degeneration resistors RE1, RE2. These amplifiers are characterized by high gains approximately equal to the ratio between the load resistance RC1 (or RC2) and the emitter degeneration resistances RE1 (or RE2).
The resistors RC1 and RC2 are generally determined by external circuits connected to the amplifier, such as via a cable or an antenna. The resistors RC1 and RC2 cannot be modified. As a consequence, it is necessary to reduce the resistances RE1 and RE2 to increment the gain.
It is well known that the amplitudes of high order harmonics depend from a decreasing function of the normalized voltage drop Vn on each emitter degeneration resistor:
      V    n    =            IC1      ·      RE1              V      T      where VT=kT/q is the thermal voltage. To summarize, the transfer characteristic of the amplifier is even more linear as the product between the collector current IC1 (or IC2) and the resistance RE1 (or RE2) increases. When values of the emitter resistance are chosen too small, this increases the current IC1 (or IC2) for a sufficiently linear transfer characteristic. Consequently, the power dissipated by the amplifier is increased.